/Engine/Source/Runtime/MovieScene/Public/EntitySystem/MovieSceneEntityManager.h
does not check serial numbers - should only be used when you are certain that another entity can not have been allocated over the top of the specified entity ID
true if the entity ID is allocated, false otherwise
bool IsAllocated ( [FMovieSceneEntityID](API\Runtime\MovieScene\EntitySystem\FMovieSceneEntityID) EntityID ) const